This invention relates to a process for dewaxing petroleum oils and fractions thereof by selectively removing normal paraffinic and other undesirable hydrocarbons from petroleum oils in which they are present in admixture with other hydrocarbons, in order to lower the pour point of such oils. More specifically, the present invention relates to a particular combination and sequence of catalytic unit processes whereby a lube oil is produced having a low pour point and a high viscosity index.
In the preparation of lube oils from hydrocarbon feeds, catalytic dewaxing processes are often combined with hydrotreating, hydrocracking and/or various solvent extraction steps to obtain products having desired properties. Typically, hydrocracking and/or solvent extraction steps are conducted prior to catalytic dewaxing to remove components such as metal-containing feed components, asphaltenes and polycyclic aromatics having properties that differ grossly from those desired. In particular, solvent extraction is conducted to remove polycyclic aromatic feed components and nitrogen-containing cyclic components, removal of the latter being particularly important in order to avoid poisoning of the catalyst in catalytic dewaxing. Hydrotreating under mild or severe conditions typically follows catalytic dewaxing operations and serves to improve oxidation stability and the nitrogen and sulfur content of the lube oil. reduce the nitrogen and sulfur content of the lube oil.
As one example of a process for producing lube oils in which a catalytic dewaxing step is included as part of a multistep process, U.S. Pat. No. 4,259,170 (Graham et al.) discloses a process that includes a combination of catalytic dewaxing and solvent dewaxing steps. According to a more specific aspect of Graham et al., the process includes a solvent extraction step prior to a dewaxing step where any suitable solvent may be used such as furfural, phenol, chlorex, nitrobenzene, or N-methyl-pyrrolidone. As a further example of a multistep process for the preparation of lube oils, U.S. Pat. No. 4,259,174 (Chen et al.) discloses a process comprising solvent extraction followed by catalytic dewaxing.
U.S. Pat. No. 4,283,272 (Garwood et al.) discloses the preparation of lube oils by a process that includes hydrocracking, catalytic dewaxing and hydrotreating steps.
U.S. Pat. No. 4,292,166 (Gorring et al.) discloses a combination process wherein a dewaxing step is carried out prior to a hydrocracking step. Specifically, a hydrocarbon oil feed selected from the group consisting of vacuum gas oils, deasphalted oils and mixtures thereof is converted to a low pour point, high VI lube base stock by first dewaxing the feed in the presence of hydrogen and a dewaxing catalyst comprising a zeolite having a Constraint Index of 1 to 12 followed by contacting the dewaxed feedstock and hydrogen with a hydroconversion catalyst comprising a platinum group metal and a zeolite having a silica to alumina ratio of at least 6. Gorring et al. also contemplate interposing a conventional hydrotreating step between the catalytic dewaxing and hydrocracking steps where the feed contains high levels of deleterious nitrogen compounds.
A combination process is also disclosed in U.S. Pat. No. 4,358,363 (Smith) wherein a fuel oil containing impurities deleterious to the catalyst is first treated with a sorbent comprising a first molecular sieve zeolite having pores with an effective diameter of at least about 5 Angstroms under sorption conditions followed by a treatment with a dewaxing catalyst comprising a second molecular sieve zeolite having pores with an effective diameter of at least about 5 Angstroms, the effective diameter of which is equal to or smaller than the effective diameter of the pores of the first molecular sieve zeolite. In a more specific aspect of the disclosure, the first and second molecular sieves have the same crystal structure wherein the Constraint Index is 1 to 12 and the dried hydrogen form crystal density is less than about 1.6 grams per cubic centimeter.
The Patentee indicates that the effectiveness of the dewaxing catalyst is increased when catalyst poisons, speculated to be basic nitrogen compounds, and oxygen and sulfur compounds, are removed. The teachings of U.S. Pat. No. 4,282,085 (O'Rear et al.) likewise appreciate the deleterious effect of nitrogen-containing impurities on ZSM-5-type crystalline aluminosilicate containing catalysts. Specifically, the Patentees disclose a process for upgrading a petroleum distillate feed with a catalyst comprising a ZSM-5-type zeolite possessing no hydrogenation activity wherein the feed has a content of nitrogencontaining impurities, calculated by weight as nitrogen, below about 5 ppm. The low-nitrogen feedstock results in a lower deactivation rate for the catalyst.
U.S. Pat. No. 4,153,540 (Gorring et al.) discloses a process for upgrading full range shale oil. More specifically, the Patentee's process involves contacting the full range shale oil with a hydrotreating catalyst and hydrogen in order to convert organic compounds of sulfur, nitrogen, oxygen, and metal. The effluent from the hydrotreater is then passed to a dewaxing zone and contacted with a dewaxing catalyst at conversion conditions calculated to hydrodewax the shale oil and convert at least 50% of the shale oil boiling above about 750.degree. F. to reaction products boiling below 750.degree. F.
Of the various solvent extraction processes, the most prevalent solvent employed is phenol. Other solvents employed include low boiling point autorefrigerative hydrocarbons, such as propane, propylene, butane, pentane, etc., liquid sulfur dioxide, furfural, and N-methyl-2-pyrrolidone (NMP). NMP is a preferred solvent because it is less toxic than the above-mentioned solvents and requires less energy to effect the extraction.
Generally, when the solvent-extracted raffinate base stocks are dewaxed with a shape selective zeolite, the viscosity index of the product oil is reduced to a greater extent than if the same stock were solvent dewaxed. This is because shape selective dewaxing catalysts reduce pour point by normal and near normal paraffin cracking which results in a concentration of low VI possessing aromatics in the product oil. Some shape selective molecular sieves are more selective than others in VI retention during dewaxing because their selectivity for normal paraffin cracking versus isoparaffin cracking is high, which results in the retention of high VI isoparaffins in the oil. For instance, even though the borosilicate molecular sieve disclosed in U.S. Pat. No. 4,269,813 (Klotz) falls in the category of high VI selective catalysts, the VI loss relative to solvent dewaxing is in the range of 8-12 VI units for phenol-extracted SAE 10 raffinate. This loss would have to be compensated for by more severe solvent extraction of aromatics which is expensive and energy consuming.
The loss in VI attributed to catalytic hydrodewaxing in comparison to solvent dewaxing is also noted in a paper entitled "Hydrodewaxing of Fuels and Lubricants using ZSM-5 type Catalysts," by R. G. Graven and J. R. Green, presented at the Australian Institute of Petroleum 1980 Congress. Therein it is mentioned that the VI for neutral distillate charge stocks dewaxed in the presence of a ZSM-5 catalyst is lower by 3 to 8 units than comparable quality solvent-dewaxed neutrals.
In a paper presented at the 1983 National Petroleum Refiners Association annual meeting entitled "Low-Temperature Performance Advantages for Hydrodewaxed Base Stocks and Products," by C. N. Rowe and J. A. Murphy, there is also pointed out that the VI differential between the catalytic dewaxing process disclosed therein and conventional solvent dewaxing ranges between 6 and 10 units for light neutral feedstocks to little or no difference for bright feedstocks.
We have observed that not all solvent raffinates can be subsequently catalytically dewaxed on an equivalent basis. In particular, the high-nitrogen-content levels, particularly basic nitrogen compounds, in certain sol- vent-extracted raffinates are believed to be responsible for the rapid deactivation of the dewaxing catalyst.
For instance, we have found NMP-extracted raffinates to be substantially more difficult to dewax over a shape-selective dewaxing catalyst, i.e., a greater deactivation rate than when phenol-extracted raffinates are catalytically dewaxed.
Thus, the prior art combination dewaxing systems and teachings can be summarized as follows. Hydrotreating is typically carried out subsequent to the catalytic dewaxing step to improve color, color stability, and to hydrogenate olefins. The deleterious effect of nitrogen on aluminosilicate-containing catalytic dewaxing catalysts is known, especially when a high nitrogen content shale oil is upgraded in a hydrodewaxing zone wherein at least 50% conversion occurs. The art is silent with respect to recognition of the detrimental effect of NMP extraction carried out prior to dewaxing on dewaxing catalyst activity, as the art considers the various solvents to be equivalent, aside from disparate cost and toxicity.
Further, while NMP extraction is desirable for the reasons cited above, the art has not recognized that NMP extraction results in a raffinate stream possessing not only NMP molecules but an increased content of other basic nitrogen compounds which are similarly detrimental to dewaxing catalyst activity. An extraction method which does not employ NMP, such as a phenol extraction method, does not produce a raffinate stream possessing the substantial basic nitrogen compound content deleterious to the dewaxing catalyst. Finally, the art is silent with respect to the VI improvement achievement in a catalytic dewaxing zone when the influent thereto is hydrotreated.
The process of the present invention obviates the rapid deactivation phenomenon described above while simultaneously, surprisingly, increasing the viscosity index (VI) and reducing the pour point of the lube stock.
Accordingly, it is a principal object of the present invention to provide a process for preparing a high grade lube base stock wherein an NMP solvent is employed in a first stage extraction step while concomitantly prolonging the activity of the dewaxing catalyst employed in the process.